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        var aa = 100;
        let fn = (a, b) => {
            /* 
            只传递了一个值 则b视为undefined 
                a = [1,2,3]
                b = undefined
            */

            b = a;
            console.log(a);
            console.log(b);

            a = 'changed';
            console.log(a);
            console.log(b);



        }

        fn(aa)

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